The proteins encoded by several type-C retroviruses have been studied. Both polycythemia- and anemia-inducing strains of defective spleen focus-forming virus (SFFV) code for 52,000 dalton glycoproteins which share determinants with the gp70's of MCF viruses. Studies using molecularly cloned SFFV DNA indicate that the expression of gp52 is correlated with the biological effects of SFFV. The helper-independent Friend murine leukemia virus (F-MuLV) induces an erythroleukemia-like disease in certain strains of newborn mice that is associated with the expression of an MCF-like virus. Susceptibility to this disease appears to be under control of genes distinct from the Fv-2 gene controlling susceptibility to SFFV. Studies with a subgenomic fragment of molecularly cloned F-MuLV DNA suggest that the env gene of F-MuLV, and by extrapolation its gene product, is associated with disease induction. Studies are in progress to determine the role that proteins encoded by SFFV and F-MuLV play in diseases induced by these viruses. In other studies, 3 isolates of feline sarcoma virus (FeSV) have been shown to code for distinct gag-x poly-proteins. Studies are currently being carried out to determine the role that these proteins play is FeSV-induced transformation.